Metallic drive shafts are an integral and necessary part of propulsion devices which operate to transmit motion to an otherwise stationary object. Examples of such drive shafts include propeller shafts for use in marine propulsion devices, such as outboard motors.
Metallic drive shafts, including propeller shafts, are generally subjected to harsh environments. In this connection, drive shafts generally comprise proximal and distal ends, one of which is typically engaged to a motion-generating means, for example, a propeller means. The area proximate the propeller means is generally exposed in use to a corrosive environment, for example, a marine environment, including sea water. The corrosive environment typically causes corrosion of the area which is proximate the propeller means.
The other of the proximal and distal ends of drive shafts is typically drivingly engaged with a motor driving means, for example, an outboard motor, for driving the shaft. Engagement of the shaft with the motor driving means is accomplished generally by frictional and/or gear engagement. Since it is often desirable to drive the shaft at high speeds, undesired wear and/or galling generally occurs in the area of the drive shaft which is proximate the motor driving means. (Galling is a severe form of adhesive wear that shows up as torn areas of the metal surface. Unless stated otherwise, reference herein to "wear" generally refers also to "galling".)
Drive shafts are generally rotatably supported at one or more positions along the length of the shaft, located between the proximal and distal ends. This support permits rotation of the shaft about its longitudinal axis as the shaft is being driven by the motor driving means. Rotatable support can be provided, for example, by roller needle bearings which frictionally contact needle bearing surfaces of the shaft. Such frictional contact can cause also wear of the shaft. The wear can be so severe as to ultimately cause failure of the propeller shaft.
Efforts have been made to produce drive shafts which can withstand harsh environments, including corrosive and wear environments. For example, drive shafts may be manufactured from stainless steel, such as austenitic stainless steel, which is generally resistant to corrosion from marine environments, including sea water.
However, stainless steel generally lacks the necessary hardness to withstand wear conditions of the type which are described above. To obtain the necessary hardness, stainless steel can be surface-hardened, for example, by exposure to surface-hardening agents. Such surface-hardening techniques include, for example, nitriding.
Although surface-hardening procedures generally produce stainless steel which has desirable wear-resistance, such procedures also diminish the inherent ability of the steel to resist corrosion. Accordingly, surface-hardened drive shafts will generally possess desirable resistance to wear, but will possess also poor resistance to corrosion.
Attempts have been made to produce drive shafts which comprise both corrosion-resistant stainless steel and wear-resistant surface-hardened stainless steel. For example, drive shafts for marine propulsion devices generally comprise a first portion (A) that is corrosion-resistant (stainless steel) and which is welded to a second portion (B) this is wear-resistant (surface-hardened stainless steel). This type of drive shaft is depicted in FIGS. 1 and 2.
However, known techniques of welding together different metals to produce drive shafts which are both corrosion- and wear-resistant suffer from various drawbacks. For example, welding operations generally require that costly secondary operations be performed to provide a usable drive shaft, including machining, heat treating and straightening operations.
Other attempts to overcome the problems associated with corrosion and wear of propeller shafts include providing a shaft which has desirable hardness and covering with a water-resistant sheath that portion of the shaft which in use is exposed to a marine environment. U.S. Pat. No. 3,952,686 describes such an arrangement, wherein the sheath comprises a sleeve, spokes, and a hub which are secured to the shaft and which are made from aluminum or an alloy thereof. This arrangement suffers from drawbacks in that access to the shaft is undesirably hindered. Moreover, additional manufacturing operations are required for the production of the protective sheath.
U.S. Pat. No. 4,604,068 discloses the use of a sacrificial anode for protecting marine propulsion device components from corrosion. This technique suffers from drawbacks in that depleted anodes must be replaced regularly and that the propeller shaft bearing housing must be removed from the lower gearcase to replace the anode.
The present invention exploits the corrosion-resistance of stainless steel and the wear-resistance of surface-hardened stainless steel and provides monolithic drive shafts which are resistant to both corrosion and wear.